Method for assembling a carbide filament incandescent lamp



g- 9 TOMITARO KUBO ETAL METHOD FOR ASSEMBLING A CARBIDE FILAMENT INCANDESCENT LAMP Filed Aug. 14, 1968 United States Patent 3,524,693 METHOD FOR ASSEMBLING A CARBIDE FILAMENT INCANDESCENT LAMP Tomitaro Kubo, Fujisawa-shi, Takao Shintani, Tokyo, Hiroyuki Murata, Yokohalna-shi, Suezo Sugaike,

Tokyo, and Sinji Yonemoto, Kanagawa-ken, Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasakishi, Japan, a corporation of Japan Filed Aug. 14, 1968, Ser. No. 752,622 Claims priority, application Japan, Aug. 17, 1967, 42/52,887; Dec. 27, 1967, 43/ 83,192 Int. Cl. H01j 9/38 US. Cl. 31621 15 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a method for assembling an incandescent lamp using a filament prepared from a sintered mass of high melting metal carbide powders, and more particularly to a method for assembling an incandescent lamp including an improved process for connecting the filament to the lead-in wires.

Heretofore, in manufacturing an incandescent lamp using a filament mainly consisting of, for example, tantalum carbide (TaC), a fine metallic tantalum wire was wound into a coil in advance and welded to lead-in wires. Thereafter there was carried out the carburization of the metallic tantalum by the heat released from the tantalum filament itself due to introduction of power therethrough or application of external heat through contact with solid carbon or in an atmosphere of hydrogen carbide, thereby to form a desired coiled filament mainly composed of tantalum carbide.

However, this process had the drawbacks that the dimensional expansion of tantalum at the time of its carburization tended to cause cracks in the carburized product, which in turn led to the breakage of the filament during manufacture of lamps or while a lamp using such filament was turned on. To prevent such failures, it might be contemplated to use a relatively thick metallic tantalum wire. In such case, however, complete carburization to obtain a desired filament posed extremely difiicult problems.

Further, carburization of a coiled metallic tantalum filament fixed to the lead-in wires had the shortcomings that carburization was likely to deform the coil, the reaction product of tantalum carbide was accompanied with the growth of crystals and the TaC filament thus formed was very liable to be broken by external shaking or impact, so that the filament required a special construction, thus unavoidably rendering the lamp bulb of complicated design.

Also in view of the aforementioned deformation due to carburization of the coiled tantalum wire fixed to the leadin wires, there has been proposed another known process which consisted in separately carburizing the coiled tantalum wire and thereafter fixing it to the lead-in wires. According to this process, powders of metallic tantalum were [formed into a rigid mass using an organic binder. Then the mass was welded to the lead-in wires using flames with padding provided in the joint. But the process still had the handicap that the tantalum powders were lost either by scattering or difiusion.

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The object of the present invention is to provide a method for assembling an incandescent lamp which is free from the aforementioned drawbacks, capable of simple construction and easy manufacture, possessed of great mechanical strength and subject to substantially no electrical and optical variations and so displays good illuminating properties.

According to the present invention, a pair of connecting wire pieces of good electrical conductivity and thermal resistance, one end of which is provided with a screwthreaded groove, are securely inserted into the apertures of an electrically insulating and preferably thermally resistant support. With the screw-threaded ends of the connecting wire pieces there are engaged a pair of sleeves of good electrical conductivity and preferably of thermal resistance, one end of which is provided with a screwthreaded groove and the other end of which is perforated with an aperture.

The sleeve aperture is filled with an adhesive agent into which is inserted one end of a filament separately prepared, consisting of a sintered mass of high melting car- [bide powders and having a narrower diameter than the aperture so as tightly to fix the filament in the sleeve aperture. To one end of each of a pair of inner leads supported by a glass stem provided with an exhaust tube and carrying a pair of outer terminals each having its one end connected to the other end of each of the inner leads, is securely caulked the other end of each of the connecting 'Wire pieces. The stem carrying the filament constructed in the aforementioned manner is sealed to a glass bulb, the interior of which is evacuated through the exhaust tube, and thereafter the exhaust tube is sealed off by melting its end.

The present invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawings, in which:

FIG. 1 shows, with a part broken away, an incandescent lamp assembled according to an embodiment of the present invention;

FIGS. 2a and 2b are enlarged views illustrative of the method of connecting the filament of an incandescent lamp; and

FIG. 3 is a schematic diagram of an electrical heating means used in the invention.

As shown in FIG. 1, the incandescent lamp assembled according to the method of the present invention comprises a glass bulb 1, outer terminals 2 and 2', inner leads 3 and 3', one end of each of which is connected to the respective outer terminals through metal to glass sealing, connecting wire pieces 4 and 4' the lower end of each of which is connected to the respective inner leads, sleeves 5 and 5' respectively fitted to the upper end of each of the connecting wire pieces 4 and 4', a filament bridging the sleeves 5 and 5', an electrically insulating support 7 for holding the connecting wire pieces at a prescribed distance and a stem through which are inserted the inner leads 3 and 3'. The sleeves 5 and 5' may be directly fitted with the inner leads 3 and 3 without the connecting wire pieces 4 and 4.

The filament 6 mainly consists of fine powders (average particle size 1.2 microns max.) of, for example, tantalum carbide TaC, also containing other minor ingredicnts such as zirconium carbide ZrC, hafnium carbide HfC or niobium carbide NbC. The filament is prepared by sintering with heat (2400 C.) in a non-oxidizing atmosphere a preshaped mass obtained by extrusion molding (a method where the primary binding agent consists of a water-soluble organic binder).

FIGS. 2a and 2b present details of the sleeves 5 and 5' for holding the filament 6 thus prepared, connecting wire pieces 4 and 4' and support 7. The sleeve 5 of FIG. 2 is made of carbonaceous matter of good electrical conductivity and thermal resistance, and assumes a substan- 3 tially cylindrical form. It has a screw-threaed groove formed inside of one end thereof, which registers with the screw-threaded groove provided at the upper end of the connecting wire piece 4.

In the interior of the sleeve above the screw-threaded groove, there is defined a conical hollow tapering toward the top. The open top end portion, through which is inserted the filament 6, has a relatively straight cavity. The sleeves 5 and 5' thus formed are engaged by screw-threads with the connecting wire pieces 4 and 4' of molybdenum or tungsten secured to a substantially circular support 7 made of a ceramic, for example, a sintered mass of magnesium oxide, and are further connected through the connecting Wire pieces to the inner leads 3 and 3' prepared from, for example, nickel-plated copper.

The void space in each of the sleeves 5 and 5' is filled with a viscous semi-paste adhesive agent 8 prepared by mixing carbon powders with, for example, coal tar and pitch as a primary solvent. The filament 6 is inserted into the adhesive agent 8 through the space of the respective sleeves 5 and 5 to be securely bonded with the sleeves.

To assure a more effective bonding between the filament 6 and sleeves 5 and 5, the filament end to be bonded may previously be dipped in a pitch liquor to the necessary extent. When, after drying, the filament 6 is inserted into the sleeves 5 and 5, the thin layer of pitch deposited on the filament end will form a strong bond with the adhesive agent 8. Addition to the adhesive agent of, for example, fine tantalum carbide TaC powders or highly conductive thermosetting material difiicultly reacting with carbon will provide good bonding. Particularly, incorporation of TaC powders in the adhesive agent 8 will be more effective to prevent the occurrence of deformations in the filament 6 or sleeves 5 and 5 bonded with the adhesive agent 8 due to excessive thermal expansion caused by elevated temperatures when an incandescent lamp is turned When the filament end is inserted into the adhesive agent 8 filled in the void space of the sleeves 5 and 5', part of the adhesive agent 8 may indeed be forced out through gaps in the upper part of the sleeves 5 and 5' depending on the volume of the filament end thus inserted. However, since said void space is shaped like a conical or tapered form, the filament end can be securely fixed, because the adhesive agent 8 is kept in a fully pressurized condition.

The sleeve 5 modified as shown in FIG. 2b is composed of heat resistant and highly conductive metal and shaped into a blind cylindrical form. Like the one of FIG. 2a, this sleeve has a screw-threaded groove formed inside of one end, which can register with the corresponding screw-threaded groove provided in the upper end of the connecting wire piece 4.

The open end of the void space of the blind sleeve 5 through which the filament 6 is inserted forms a straight cavity. In fabricating an incandescent lamp bulb, the blind sleeves 5 and 5' constructed in the aforementioned manner are engaged like those of FIG. 2a with the upper ends of the connecting wire pieces 4 and 4'.

The void space of each of the sleeves 5 and 5' modified as described above is filled with powders 8 of metal having a lower melting point than, or a eutectic point with, the metal material of the sleeves 5 and 5, namely, at least one of the group consisting of, for example, tungsten, molybdenum, manganese, iron, tantalum, niobium, titanium, platinum rhodium, iridium, chromium, nickel, copper, aluminum and zinc. On the charged metal powders is placed a ring 12 which is intended to be used as a medium in compressing the adhesive agent filled in the sleeve aperture, and the filament 6 is inserted into the metal powders 8 through the open end of the sleeves 5 and 5'. The metal powders are melted by locally heating the side of the sleeves S and 5' thereby to bond the filament therewith.

To ensure more eflfective bonding between the filament 6 and sleeves 5 and 5', it is advisable to plate the filament end in advance to the required extent with metal (of the same kind as that used as an adhesive agent) and insert it into the adhesive agent 8 filled in the sleeves 5 and 5'. Then the thin metal film plated on the filament end will provide strong bonding with the metal powders 8. It is also permissible to add to the metal powders 8, for example, powders of tantalum carbide TaC or highly conductive thermosetting material. When the filament end is inserted into the metal powders 8 fitted in the void space of the sleeves 5 and 5, part of the metal powders 8 may indeed be forced out through gaps in the upper part of the sleeves 5 and 5' depending on the volume of the fila ment end inserted. However, if, in this case, the void space of the sleeves 5 and 5' is shaped into a tapered or conical form, the adhesive agent 8 of metal powders will be fully pressed when the ring 12 is forced down thereon, so that the adhesive agent is securely set in place or kept in a well-charged condition.

When used in an incandescent lamp, a filament prepared as described above is not likely to be broken under external shaking or impact due to its great mechanical strength. Further it assures good contact with the connecting wire pieces through the sleeves, so that it is not adversely affected in any way with respect to electrical contact, and consequently illuminating properties.

There will now be described the present invention by reference to the examples which follow. It will be understood that they are offered only by way of illustration and are not intended to restrict the scope of the invention.

EXAMPLE 1 At one end of each of a pair of connecting wire pieces 4 and 4 made of molybdenum was formed a screwthreaded groove. The connecting wire pieces 4 and 4' thus processed were fixed in place by being inserted through the apertures 11 of a support 7 prepared from a sintered mass of magnesium oxide. In the lower part of the apertures 11 of the support 7 through which the connecting wire pieces were to be inserted there was filled an inorganic adhesive agent 10 composed of a mixture of, for example, 1 weight part of sodium silicate, 1.5 Weight parts of calcined magnesia and 0.75 Weight part of talc. The upper part of the apertures 11 may also be filled with the inorganic adhesive agent 10. Thereafter the adhesive agent 10 was securely set in place by allowing to stand overnight for natural drying, or by purposely drying it at a temperature of 50 to C. for about 4 hours. After the connecting wire pieces 4 and 4' were fixed by being bonded with the inorganic adhesive agent 10, the sleeves 5 and 5 were screwed to the respective upper ends thereof as shown in FIG. 2a. The void space of each of the sleeves 5 and 5 was fully filled, while heating to to C., with an adhesive agent (prepared by first mixing 40 percent by weight of 300 mesh (JIS) pass graphite powders and 60 percent by weight of tantalum carbide powders having an average particle size of 1.2 microns max. and then adding 7 to 15 percent by weight of a 1:1 mixture of coal tar and pitch made into a viscous mass of 200 centipoises with benzol, on the basis of the total weight of the aforesaid mixed powders, followed by kneading). The filling of the adhesive agent 8 in the void space of the sleeves 5 and 5' may be carried out before they are screwed to the connecting rwire pieces. Both ends of the filament 6 were dipped in a coal tar solution and, after removal therefrom, dried at 300 C. for 2 hours, and then inserted into the adhesive agent 8 already filled in the void spaces of the sleeves 5 and 5'. When the sleeves 5 and 5' were cooled after insertion of the filament 6 the adhesive agent 8 filled in the sleeve apertures was solidified.

To carry out the baking of the adhesive agent 8 after its solidification, the assembly of the filament, sleeves and connecting wire pieces were vertically set on the carrier stand of a baking furnace (not shown). Baking was conducted by raising the temperature of the furnace stepwise as follows: from room temperature to 200 C. at the rate of 80 C./hr., from 200 to 400 C. at 100 C./hr., from 400 to 1,200" C. at 150 C./hr. and finally 1 hr. at 1,200 C. Thus the adhesive agent 8 was baked to hardness unfailingly and satisfactorily. For this baking the atmosphere within the baking furnace consisted of a non-oxidizing gas, for example, hydrogen, argon or nitrogen which was introduced at the rate of 1,000 to 1,500 l./hr. The connecting wire pieces 4 and 4 contacting the filament 6 through the sleeves 5 and 5' were fitted by, for example, caulking to the upper ends of the inner leads 3 and 3 secured to the glass stem 9 so as to form an electrode mount. The mount was sealed to the glass lamp bulb 1 and the exhaust tube was sealed off by melting its end after the prescribed evacuation of the interior of the lamp bulb 1 thereby to finish an incandescent lamp.

As mentioned above, the filament 6 separately sintered and connected to the connecting wire pieces by being securely bonded with the sleeves 5 and 5' has extremely great strength to mechanical vibrations and provides satisfactory electrical connection, so that the filament enables an incandescent lamp to exhibit excellent illumination properties free from poor electrical contact, current variations and irregular projection of light.

EXAMPLE 2 At one end of each of a pair of connecting wire pieces 4 and 4' made of molybdenum was formed a screwthreaded groove. The connecting wire pieces 4 and 4' thus processed were fixed in place by being inserted through the apertures 11 of a support 7 prepared from a sintered mass of magnesium oxide. In the lower part of the apertures 11 of the support 7 through which the connecting wire pieces were to be inserted there was filled an inorganic adhesive agent 10 composed of a mixture of, for example, 1 weight part of sodium silicate, 1.5 weight parts of calcined magnesia and 0.75 weight part of tale. The upper part of the apertures 11 may also be filled with the inorganic adhesive agent 10. Thereafter the adhesive agent 10 was securely set in place by allowing to stand overnight for natural drying, or by purposely drying it at a temperature of 50 C. to 80 C. for about 4 hours. After the connecting wire pieces 4 and 4 were fixed by being bonded with the inorganic adhesive agent 10, the blind sleeves 5 and 5 made of molybdenum were screwed to the respective upper ends thereof as shown in FIG. 2b. The void space of each of the sleeves 5 and 5' was filled with metal powders 8 (325 mesh (1 IS) pass mixed powders consisting of 50 percent by weight of molybdenum and 50 percent by weight of chromium) and the end of the filament 6 was inserted into the charged metal powders. Thereafter a pair of rings 12 made of molybdenum which had previously been fitted to the filament 6 by passing it through the aperture of the respective rings were pressed down on the inner upper part of the respective sleeves 5 and 5' so as to compress the charged metal powders 8. The assembled filament 6 including the rings 12, sleeves 5 and 5' and connecting wire pieces 4 and 4 were introduced into an electrical heating device 13 of FIG. 3 containing an inert atmosphere. Power was introduced across the electrodes 14 and 14 to heat a carbon heating element 15 for the local heating of the sleeves 5 and 5'. The assembly was allowed to stand at a temperature of about 2,000 C. for 3 minutes. The heating element 15 had a through hole 16 to receive either of the sleeves 5 and 5' the aforementioned heating operation caused the metal powders to be melted and upon solidification to be unfailingly and satisfactorily bonded with the filament. The connecting wire pieces 4 and 4 contacting the filament 6 through the sleeves 5 and 5' were fitted by, for example, caulking to the upper ends of the inner leads 3 and 3 secured to the glass stem 9 so as to form an electrode mount. The mount was sealed to the glass lamp bulb 1 and the exhaust tube was sealed off by melting its end after the prescribed evacuation of the interior of the lamp bulb 1 thereby to finish an incandescent lamp. The incandescent lamp thus fabricated displayed as good properties as that of Example 1.

EXAMPLE 3 The sleeves and rings as shown in FIG. 2b consisted of nickel, the filament was composed of a sintered mass of a mixture of tantalum carbide TaC and hafnium carbide HfC, and the charged metal powders comprised 325 mesh (JIS) pass mixed powders of percent by weight of copper and 20 percent by weight of zinc. The sleeve was heated to 1,000" C. by an electrical furnace in an inert atmosphere and then allowed to cool after being kept at that temperature for 3 minutes.

In the incandescent lamp thus assembled, an adhesive agent of metal powders was so fully diffused as to leave substantially no void spaces between the filament and sleeves so that they were bonded together tightly. Consequently the assembly of the filament and sleeves was shown to be free from any poor electrical contact and extremely strong to mechanical impacts. It also had as good illuminating properties as in Example 1 and exhibited no current variation.

It will be apparent that the present invention is not limited to the aforementioned examples. For instance, the powdered material of the filament may include not only tantalum carbide, but also any other high melting metal carbides. Also the metal powders for bonding the filament with the sleeves and the inorganic adhesive agents for bonding the connecting wire pieces with the support are not limited to those used in the examples, but their compositions may be suitably varied.

The adhesive agent 8 used in Example 1 for bonding the filament 6 with the respective sleeves 5 and 5' may be replaced by another means which comprises drilling a female screw hole on that side of the respective sleeves 5 and S contacted by the filament and tightly pressing the inserted filament to the hole wall by fitting a male screw member made of carbon into the female screw.

Examples 2 and 3 relate to the case where separate blind sleeves 5 and 5' were respectively screwed to separate connecting wire pieces. However, the sleeves may be integrally formed with the connecting rwire pieces thereby to fit the filament into an aperture provided in the upper part of the respective integral bodies.

What is claimed is:

1. A method for assembling a carbide filament incandescent lamp, comprising the steps of inserting a pair of electrically conductive connecting wire pieces each having a screw-threaded groove formed at one end into a pair of apertures of an electrically insulating support and fixing the connecting wire pieces to the support; fitting a pair of sleeves each having a screw-threaded groove formed inside said sleeve at one end with said one end of the respective connecting wire pieces by means of screw engagement, said sleeves each having an aperture perforated at the other end; filling the sleeve aperture with an adhesive agent and inserting into the adhesive agent the end of a carbide filament separately prepared and having a narrower diameter than said aperture thereby to fix the filament end in the sleeve aperture; caulking the other end of the respective connecting wire pieces to one end of a pair of inner leads supported by a glass stem provided with an exhaust tube and carrying a pair of outer terminals each having its one end conneted to the other end of each of the inner leads; sealing to a glass lamp bulb the glass stem carrying the filament thus constructed; evacuating the interior of the glass bulb through the exhaust tube; and finally sealing oif the exhaust tube by melting its end.

2. A method of claim 1 wherein the connecting wire pieces are fixed to the support by being bonded with an inorganic bonding agent filled in the space of the support apertures.

3. A method of claim 2 wherein each of the sleeves consists of a carbon tube.

4. A method of claim 3 wherein the sleeve aperture is a conical hollow tapering toward the top of said sleeve, with the open top end portion forming a straight cavity.

5. A method of claim 4 wherein said adhesive agent to be filled in the sleeve aperture is prepared by first mixing 40 percent by weight of 300 mesh (JIS) pass graphite powders and 60 percent by weight of tantalum carbide powders having an average particle size of 1.2 microns maximum and then adding 7 to 15 percent by weight of a 1:1 mixture of coal tar and pitch made into a viscous mass of 200 centipoises with benzol, on the basis of the total weight of the aforesaid mixed powders, followed by kneading.

6. A method of claim 5 wherein said adhesive agent is filled into the sleeve aperture while heating to 100 to 120 C.

7. A method of claim 6 wherein said adhesive agent with the end of the filament inserted therein is cooled to be solidified.

8. A method of claim 7 wehrein said adhesive agent solidified is baked to hardness in an atmosphere of a non-oxidizin g gas.

9. A method of claim 3 wherein previous to insertion of the filament end into the adhesive agent, said filament end is dipped into a body of coal tar, withdrawn therefrom and is dried at 300 C. for 2 hours.

10. A method of claim 2 wherein each of the sleeves is composed of a highly conductive and heat resistant metal.

11. A method of claim 10 wherein the sleeve aperture is a blind straight cavity.

12. A method of claim 11 wherein said adhesive agent to be filled in the sleeve aperture comprises powders of at least one metal material having a melting point not exceeding that of the metal material of said sleeve.

13. A method of claim 12 wherein said adhesive agent filled in each of the sleeve apertures is compressed by a ring having been previously fitted to the filament.

14. A method of claim 13 wherein said adhesive agent with the end of the filament inserted therein is heated to be melted and then cooled to be solidified.

15. A method of claim 10 wherein previous to insertion of the filament end into the adhesive agent, said filament end is plated with the same kind of metal as said adhesive agent.

References Cited UNITED STATES PATENTS 2,482,447 9/ 1949 Van Liempt et al. 316-24 3,316,049 4/1967 Brown 31621 3,408,719 11/1968 Van Sickler et a1. 2925.15 3,460,219 8/1969 Shiragaki 29-25.13

JOHN F. CAMPBELL, Primary Examiner R. J. CRAIG, Assistant Examiner US. Cl. X.R. 

